Multi-parameter arrhythmia discrimination

ABSTRACT

An arrhythmia discrimination device and method involves receiving electrocardiogram signals and non-electrophysiologic signals at subcutaneous locations. Both the electrocardiogram-signals and non-electrophysiologic signals are used to discriminate between normal sinus rhythm and an arrhythmia. An arrhythmia may be detected using electrocardiogram signals, and verified using the non-electrophysiologic signals. A detection window may be initiated in response to receiving the electrocardiogram signal, and used to determine whether the non-electrophysiologic signal is received at a time falling within the detection window. Heart rates may be computed based on both the electrocardiogram signals and non-electrophysiologic signals. The rates may be used to discriminate between normal sinus rhythm and arrhythmia, and used to determining absence of an arrhythmia.

RELATED APPLICATIONS

This application claims the benefit of Provisional Patent ApplicationSer. No. 60/462,272, filed on Apr. 11, 2003, to which priority isclaimed pursuant to 35 U.S.C. §119(e) and which is hereby incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to implantable cardiacmonitoring and stimulation devices and, more particularly, tomulti-parameter arrhythmia discrimination using electrocardiograminformation and non-electrophysiological cardiac activity information.

BACKGROUND OF THE INVENTION

The healthy heart produces regular, synchronized contractions. Rhythmiccontractions of the heart are normally controlled by the sinoatrial (SA)node, which is a group of specialized cells located in the upper rightatrium. The SA node is the normal pacemaker of the heart, typicallyinitiating 60–100 heartbeats per minute. When the SA node is pacing theheart normally, the heart is said to be in normal sinus rhythm.

If the heart's electrical activity becomes uncoordinated or irregular,the heart is denoted to be arrhythmic. Cardiac arrhythmia impairscardiac efficiency and can be a potential life-threatening event.Cardiac arrhythmias have a number of etiological sources, includingtissue damage due to myocardial infarction, infection, or degradation ofthe heart's ability to generate or synchronize the electrical impulsesthat coordinate contractions.

Bradycardia occurs when the heart rhythm is too slow. This condition maybe caused, for example, by impaired function of the SA node, denotedsick sinus syndrome, or by delayed propagation or blockage of theelectrical impulse between the atria and ventricles. Bradycardiaproduces a heart rate that is too slow to maintain adequate circulation.

When the heart rate is too rapid, the condition is denoted tachycardia.Tachycardia may have its origin in either the atria or the ventricles.Tachycardias occurring in the atria of the heart, for example, includeatrial fibrillation and atrial flutter. Both conditions arecharacterized by rapid contractions of the atria. Besides beinghemodynamically inefficient, the rapid contractions of the atria mayalso adversely affect the ventricular rate.

Ventricular tachycardia occurs, for example, when electrical activityarises in the ventricular myocardium at a rate more rapid than thenormal sinus rhythm. Ventricular tachycardia can quickly degenerate intoventricular fibrillation. Ventricular fibrillation is a conditiondenoted by extremely rapid, uncoordinated electrical activity within theventricular tissue. The rapid and erratic excitation of the ventriculartissue prevents synchronized contractions and impairs the heart'sability to effectively pump blood to the body, which is a fatalcondition unless the heart is returned to sinus rhythm within a fewminutes.

Implantable cardiac rhythm management systems have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically include one or more leads and circuitry to sense signals fromone or more interior and/or exterior surfaces of the heart. Such systemsalso include circuitry for generating electrical pulses that are appliedto cardiac tissue at one or more interior and/or exterior surfaces ofthe heart. For example, leads extending into the patient's heart areconnected to electrodes that contact the myocardium for sensing theheart's electrical signals and for delivering pulses to the heart inaccordance with various therapies for treating arrhythmias.

Typical Implantable cardioverter/defibrillators (ICDs) include one ormore endocardial leads to which at least one defibrillation electrode isconnected. Such ICDs are capable of delivering high-energy shocks to theheart, interrupting the ventricular tachyarrhythmia or ventricularfibrillation, and allowing the heart to resume normal sinus rhythm. ICDsmay also include pacing functionality.

Although ICDs are very effective at preventing Sudden Cardiac Death(SCD), most people at risk of SCD are not provided with implantabledefibrillators. Primary reasons for this unfortunate reality include thelimited number of physicians qualified to perform transvenouslead/electrode implantation, a limited number of surgical facilitiesadequately equipped to accommodate such cardiac procedures, and alimited number of the at-risk patient population that may safely undergothe required endocardial or epicardial lead/electrode implant procedure.

SUMMARY OF THE INVENTION

The present invention is directed to cardiac monitoring and/orstimulation methods and systems that, in general, provide transthoracicmonitoring, defibrillation therapies, pacing therapies, or a combinationof these capabilities. Embodiments of the present invention are directedto subcutaneous cardiac monitoring and/or stimulation methods andsystems that detect and/or treat cardiac activity or arrhythmias.

An embodiment of the invention is directed to an arrhythmiadiscrimination method that involves sensing electrocardiogram signals ata subcutaneous non-intrathoracic location. The electrocardiogram signalsmay include a cardiac signal and one or both of noise andelectrocardiographic artifacts. Signals associated with anon-electrophysiological cardiac source are also received. A sensedelectrocardiogram signal is verified to be a cardiac signal using anon-electrophysiological signal. A cardiac arrhythmia is detected usingone or both of the sensed electrocardiogram signal and the verifiedcardiac signal. Treatment of the cardiac arrhythmia is withheld if thesensed signal is not verified to be the cardiac signal.

An arrhythmia may be detected using the electrocardiogram signals, andthe presence of the arrhythmia may be verified or refuted using thenon-electrophysiologic signals. Temporal relationships between theelectrocardiogram signals and non-electrophysiologic signals may bedetermined. A detection window may be initiated in response to receivingthe electrocardiogram signal, and used to determine whether thenon-electrophysiologic signal is received at a time falling within thedetection window.

Heart rates may be computed based on both a succession ofelectrocardiogram signals and a succession of non-electrophysiologicsignals. The rates may be used to discriminate between normal sinusrhythm and the arrhythmia. The rates may be compared with arrhythmiathresholds, and used to determine absence of an arrhythmia, such as inresponse to a first rate exceeding a first arrhythmia threshold and asecond rate failing to exceed a second arrhythmia threshold. Thepresence of an arrhythmia may be determined using a morphology of theelectrocardiogram signals, and then verified using thenon-electrophysiologic signals. Examples of non-electrophysiologicsignals include heart sound signals, subsonic acoustic signalsindicative of cardiac activity, pulse pressure signals, impedancesignals indicative of cardiac activity, and pulse oximetry signals.

In another embodiment of the present invention, defibrillation therapydelivery may be inhibited in response to detecting an arrhythmia usingthe electrocardiogram signals but not detecting the arrhythmia using thenon-electrophysiologic signal. A method of sensing an arrhythmia andinhibiting therapy may involve sensing an electrocardiogram signal at asubcutaneous non-intrathoracic location. A detection window may bedefined with a start time determined from the electrocardiogram signal.A signal associated with a non-electrophysiological cardiac source maybe received and evaluated within the detection window. The presence ornon-presence of a cardiac arrhythmia may be determined using theelectrocardiogram signal, and confirmed by the presence of the cardiacarrhythmia as detected by the non-electrophysiological cardiac signal.The start time of a detection window used for confirmation may beassociated with an inflection point of the electrocardiogram signal,such as a maxima or a minima. A correlation may be performed between theelectrocardiogram signal and the non-electrophysiological cardiacsignal.

An embodiment of the present invention is directed to an implantablecardiac device including a housing and an electrode arrangementconfigured for subcutaneous non-intrathoracic placement. Detectioncircuitry is provided in the housing and coupled to the electrodearrangement. The detection circuitry is configured to detectelectrocardiogram signals comprising a cardiac signal and one or both ofnoise and electrocardiographic artifacts. A sensor configured to sensenon-electrophysiologic signals associated with anon-electrophysiological cardiac source is coupled to the detectioncircuitry. A processor is provided in the housing and coupled to thedetection circuitry, sensor, and energy delivery circuitry, anddiscriminates between normal sinus rhythm and arrhythmia using theelectrocardiogram and non-electrophysiologic signals. The processorverifies that the sensed electrocardiogram signal is a cardiac signalusing the non-electrophysiological signal. The processor withholdstreatment of the cardiac arrhythmia if the sensed signal is not verifiedto include the cardiac signal.

The energy delivery circuitry may include one or both of defibrillationtherapy circuitry and pacing therapy circuitry. The sensor may beprovided in or on the housing, and/or in or on a lead coupled to thehousing. Appropriate sensors include an accelerometer, a microphone, anacoustic transducer, a blood-flow transducer, photoplethysmographycircuitry, and a pulse oximeter.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views of a transthoracic cardiac sensing and/orstimulation device as implanted in a patient in accordance with anembodiment of the present invention;

FIG. 1C is a block diagram illustrating various components of atransthoracic cardiac sensing and/or stimulation device in accordancewith an embodiment of the present invention;

FIG. 1D is a block diagram illustrating various processing and detectioncomponents of a transthoracic cardiac sensing and/or stimulation devicein accordance with an embodiment of the present invention;

FIG. 2 is a diagram illustrating components of a transthoracic cardiacsensing and/or stimulation device including an electrode array inaccordance with an embodiment of the present invention;

FIG. 3 is a pictorial diagram of a carotid pulse waveform, aphonocardiogram (PCG) waveform, an electrocardiogram (ECG) waveform, anda filtered transthoracic impedance signal for two consecutiveheartbeats;

FIG. 4 is a graph illustrating two consecutive PQRS complexes and theirassociated pseudo accelerometer signals, and a detection window forcorrelation of the signals in accordance with an embodiment of thepresent invention;

FIG. 5 is a flow chart illustrating methods of multi-parameterarrhythmia discrimination in accordance with the present invention; and

FIG. 6 is a flow chart illustrating a method of multi-parameterarrhythmia discrimination in accordance with the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings, which form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

An implanted device according to the present invention may include oneor more of the features, structures, methods, or combinations thereofdescribed hereinbelow. For example, a cardiac monitor or a cardiacstimulator may be implemented to include one or more of the advantageousfeatures and/or processes described below. It is intended that such amonitor, stimulator, or other implanted or partially implanted deviceneed not include all of the features described herein, but may beimplemented to include selected features that provide for uniquestructures and/or functionality. Such a device may be implemented toprovide a variety of therapeutic or diagnostic functions.

In general terms, a cardiac signal discrimination arrangement and methodmay be used with a subcutaneous cardiac monitoring and/or stimulationdevice. One such device is an implantable transthoracic cardiac sensingand/or stimulation (ITCS) device that may be implanted under the skin inthe chest region of a patient. The ITCS device may, for example, beimplanted subcutaneously such that all or selected elements of thedevice are positioned on the patient's front, back, side, or other bodylocations suitable for sensing cardiac activity and delivering cardiacstimulation therapy. It is understood that elements of the ITCS devicemay be located at several different body locations, such as in thechest, abdominal, or subclavian region with electrode elementsrespectively positioned at different regions near, around, in, or on theheart.

The primary housing (e.g., the active or non-active can) of the ITCSdevice, for example, may be configured for positioning outside of therib cage at an intercostal or subcostal location, within the abdomen, orin the upper chest region (e.g., subclavian location, such as above thethird rib). In one implementation, one or more electrodes may be locatedon the primary housing and/or at other locations about, but not indirect contact with the heart, great vessel or coronary vasculature.

In another implementation, one or more leads incorporating electrodesmay be located in direct contact with the heart, great vessel orcoronary vasculature, such as via one or more leads implanted by use ofconventional transvenous delivery approaches. In a furtherimplementation, for example, one or more subcutaneous electrodesubsystems or electrode arrays may be used to sense cardiac activity anddeliver cardiac stimulation energy in an ITCS device configurationemploying an active can or a configuration employing a non-active can.Electrodes may be situated at anterior and/or posterior locationsrelative to the heart. Examples of useful subcutaneous electrodes,electrode arrays, and orientations of same are described in commonlyowned U.S. patent application Ser. No. 10/738,608 entitled “NoiseCanceling Cardiac Electrodes,” filed Dec. 17, 2003, and U.S. patentapplication Ser. No. 10/465,520 filed Jun. 19, 2003 entitled “MethodsAnd Systems Involving Subcutaneous Electrode Positioning Relative To AHeart”, which are hereby incorporated herein by reference.

Certain configurations illustrated herein are generally described ascapable of implementing various functions traditionally performed by animplantable cardioverter/defibrillator (ICD), and may operate innumerous cardioversion/defibrillation modes as are known in the art.Examples of ICD circuitry, structures and functionality, aspects ofwhich may be incorporated in an ITCS device in accordance with thepresent invention, are disclosed in commonly owned U.S. Pat. Nos.5,133,353; 5,179,945; 5,314,459; 5,318,597; 5,620,466; and 5,662,688,which are hereby incorporated herein by reference in their respectiveentireties.

In particular configurations, systems and methods may perform functionstraditionally performed by pacemakers, such as providing various pacingtherapies as are known in the art, in addition tocardioversion/defibrillation therapies. Examples of pacemaker circuitry,structures and functionality, aspects of which may be incorporated in anITCS device in accordance with the present invention, are disclosed incommonly owned U.S. Pat. Nos. 4,562,841; 5,284,136; 5,376,106;5,036,849; 5,540,727; 5,836,987; 6,044,298; and 6,055,454, which arehereby incorporated herein by reference in their respective entireties.It is understood that ITCS device configurations may provide fornon-physiologic pacing support in addition to, or to the exclusion of,bradycardia and/or anti-tachycardia pacing therapies.

An ITCS device in accordance with the present invention may implementdiagnostic and/or monitoring functions as well as provide cardiacstimulation therapy. Examples of cardiac monitoring circuitry,structures and functionality, aspects of which may be incorporated in anITCS device in accordance with the present invention, are disclosed incommonly owned U.S. Pat. Nos. 5,313,953; 5,388,578; and 5,411,031, whichare hereby incorporated herein by reference in their respectiveentireties.

An ITCS device may be used to implement various diagnostic functions,which may involve performing rate-based, pattern and rate-based, and/ormorphological tachyarrhythmia discrimination analyses. Subcutaneous,cutaneous, and/or external sensors may be employed to acquirephysiologic and non-physiologic information for purposes of enhancingtachyarrhythmia detection and termination. It is understood thatconfigurations, features, and combination of features described in thepresent disclosure may be implemented in a wide range of implantablemedical devices, and that such embodiments and features are not limitedto the particular devices described herein.

Referring now to FIGS. 1A and 1B of the drawings, there is shown aconfiguration of a transthoracic cardiac sensing and/or stimulation(ITCS having components implanted in the chest region of a patient atdifferent locations. In the particular configuration shown in FIGS. 1Aand 1B, the ITCS device includes a housing 102 within which variouscardiac sensing, detection, processing, and energy delivery circuitrymay be housed. It is understood that the components and functionalitydepicted in the figures and described herein may be implemented inhardware, software, or a combination of hardware and software. It isfurther understood that the components and functionality depicted asseparate or discrete blocks/elements in the figures may be implementedin combination with other components and functionality, and that thedepiction of such components and functionality in individual or integralform is for purposes of clarity of explanation, and not of limitation.

Communications circuitry is disposed within the housing 102 forfacilitating communication between the ITCS device and an externalcommunication device, such as a portable or bed-side communicationstation, patient-carried/worn communication station, or externalprogrammer, for example. The communications circuitry may alsofacilitate unidirectional or bidirectional communication with one ormore external, cutaneous, or subcutaneous physiologic or non-physiologicsensors. The housing 102 is typically configured to include one or moreelectrodes (e.g., can electrode and/or indifferent electrode). Althoughthe housing 102 is typically configured as an active can, it isappreciated that a non-active can configuration may be implemented, inwhich case at least two electrodes spaced apart from the housing 102 areemployed.

In the configuration shown in FIGS. 1A and 1B, a subcutaneous electrode104 may be positioned under the skin in the chest region and situateddistal from the housing 102. The subcutaneous and, if applicable,housing electrode(s) may be positioned about the heart at variouslocations and orientations, such as at various anterior and/or posteriorlocations relative to the heart. The subcutaneous electrode 104 iscoupled to circuitry within the housing 102 via a lead assembly 106. Oneor more conductors (e.g., coils or cables) are provided within the leadassembly 106 and electrically couple the subcutaneous electrode 104 withcircuitry in the housing 102. One or more sense, sense/pace ordefibrillation electrodes may be situated on the elongated structure ofthe electrode support, the housing 102, and/or the distal electrodeassembly (shown as subcutaneous electrode 104 in the configuration shownin FIGS. 1A and 1B).

In one configuration, the lead assembly 106 is generally flexible andhas a construction similar to conventional implantable, medicalelectrical leads (e.g., defibrillation leads or combineddefibrillation/pacing leads). In another configuration, the leadassembly 106 is constructed to be somewhat flexible, yet has an elastic,spring, or mechanical memory that retains a desired configuration afterbeing shaped or manipulated by a clinician. For example, the leadassembly 106 may incorporate a gooseneck or braid system that may bedistorted under manual force to take on a desired shape. In this manner,the lead assembly 106 may be shape-fit to accommodate the uniqueanatomical configuration of a given patient, and generally retains acustomized shape after implantation. Shaping of the lead assembly 106according to this configuration may occur prior to, and during, ITCSdevice implantation.

In accordance with a further configuration, the lead assembly 106includes a rigid electrode support assembly, such as a rigid elongatedstructure that positionally stabilizes the subcutaneous electrode 104with respect to the housing 102. In this configuration, the rigidity ofthe elongated structure maintains a desired spacing between thesubcutaneous electrode 104 and the housing 102, and a desiredorientation of the subcutaneous electrode 104/housing 102 relative tothe patient's heart. The elongated structure may be formed from astructural plastic, composite or metallic material, and includes, or iscovered by, a biocompatible material. Appropriate electrical isolationbetween the housing 102 and subcutaneous electrode 104 is provided incases where the elongated structure is formed from an electricallyconductive material, such as metal.

In one configuration, the rigid electrode support assembly and thehousing 102 define a unitary structure (e.g., a single housing/unit).The electronic components and electrode conductors/connectors aredisposed within or on the unitary ITCS device housing/electrode supportassembly. At least two electrodes are supported on the unitary structurenear opposing ends of the housing/electrode support assembly. Theunitary structure may have an arcuate or angled shape, for example.

According to another configuration, the rigid electrode support assemblydefines a physically separable unit relative to the housing 102. Therigid electrode support assembly includes mechanical and electricalcouplings that facilitate mating engagement with correspondingmechanical and electrical couplings of the housing 102. For example, aheader block arrangement may be configured to include both electricaland mechanical couplings that provide for mechanical and electricalconnections between the rigid electrode support assembly and housing102. The header block arrangement may be provided on the housing 102 orthe rigid electrode support assembly. Alternatively, amechanical/electrical coupler may be used to establish mechanical andelectrical connections between the rigid electrode support assembly andhousing 102. In such a configuration, a variety of different electrodesupport assemblies of varying shapes, sizes, and electrodeconfigurations may be made available for physically and electricallyconnecting to a standard ITCS device housing 102.

It is noted that the electrodes and the lead assembly 106 may beconfigured to assume a variety of shapes. For example, the lead assembly106 may have a wedge, chevron, flattened oval, or a ribbon shape, andthe subcutaneous electrode 104 may include a number of spacedelectrodes, such as an array or band of electrodes. Moreover, two ormore subcutaneous electrodes 104 may be mounted to multiple electrodesupport assemblies 106 to achieve a desired spaced relationship amongstsubcutaneous electrodes 104.

An ITCS device may incorporate circuitry, structures and functionalityof the subcutaneous implantable medical devices disclosed in commonlyowned U.S. Pat. Nos. 5,203,348; 5,230,337; 5,360,442; 5,366,496;5,397,342; 5,391,200; 5,545,202; 5,603,732; and 5,916,243, which arehereby incorporated herein by reference in their respective entireties.

FIG. 1C is a block diagram depicting various components of an ITCSdevice in accordance with one configuration. According to thisconfiguration, the ITCS device incorporates a processor-based controlsystem 205 which includes a micro-processor 206 coupled to appropriatememory (volatile and non-volatile) 209, it being understood that anylogic-based control architecture may be used. The control system 205 iscoupled to circuitry and components to sense, detect, and analyzeelectrical signals produced by the heart and deliver electricalstimulation energy to the heart under predetermined conditions to treatcardiac arrhythmias. In certain configurations, the control system 205and associated components also provide pacing therapy to the heart. Theelectrical energy delivered by the ITCS device may be in the form of lowenergy pacing pulses or high-energy pulses for cardioversion ordefibrillation.

Cardiac signals are sensed using the subcutaneous electrode(s) 214 andthe can or indifferent electrode 207 provided on the ITCS devicehousing. Cardiac signals may also be sensed using only the subcutaneouselectrodes 214, such as in a non-active can configuration. As such,unipolar, bipolar, or combined unipolar/bipolar electrode configurationsas well as multi-element electrodes and combinations of noise cancelingand standard electrodes may be employed. The sensed cardiac signals arereceived by sensing circuitry 204, which includes sense amplificationcircuitry and may also include filtering circuitry and ananalog-to-digital (A/D) converter. The sensed cardiac signals processedby the sensing circuitry 204 may be received by noise reductioncircuitry 203, which may further reduce noise before signals are sent tothe detection circuitry 202.

Noise reduction circuitry 203 may also be incorporated after sensingcircuitry 202 in cases where high power or computationally intensivenoise reduction algorithms are required. The noise reduction circuitry203, by way of amplifiers used to perform operations with the electrodesignals, may also perform the function of the sensing circuitry 204.Combining the functions of sensing circuitry 204 and noise reductioncircuitry 203 may be useful to minimize the necessary componentry andlower the power requirements of the system.

In the illustrative configuration shown in FIG. 1C, the detectioncircuitry 202 is coupled to, or otherwise incorporates, noise reductioncircuitry 203. The noise reduction circuitry 203 operates to improve thesignal-to-noise ratio (SNR) of sensed cardiac signals by removing noisecontent of the sensed cardiac signals introduced from various sources.Typical types of transthoracic cardiac signal noise includes electricalnoise and noise produced from skeletal muscles, for example.

Detection circuitry 202 typically includes a signal processor thatcoordinates analysis of the sensed cardiac signals and/or other sensorinputs to detect cardiac arrhythmias, such as, in particular,tachyarrhythmia. Rate based and/or morphological discriminationalgorithms may be implemented by the signal processor of the detectioncircuitry 202 to detect and verify the presence and severity of anarrhythmic episode. Examples of arrhythmia detection and discriminationcircuitry, structures, and techniques, aspects of which may beimplemented by an ITCS device in accordance with the present invention,are disclosed in commonly owned U.S. Pat. Nos. 5,301,677 and 6,438,410,which are hereby incorporated herein by reference in their respectiveentireties.

The detection circuitry 202 communicates cardiac signal information tothe control system 205. Memory circuitry 209 of the control system 205contains parameters for operating in various sensing, defibrillation,and, if applicable, pacing modes, and stores data indicative of cardiacsignals received by the detection circuitry 202. The memory circuitry209 may also be configured to store historical ECG and therapy data,which may be used for various purposes and transmitted to an externalreceiving device as needed or desired.

In certain configurations, the ITCS device may include diagnosticscircuitry 210. The diagnostics circuitry 210 typically receives inputsignals from the detection circuitry 202 and the sensing circuitry 204.The diagnostics circuitry 210 provides diagnostics data to the controlsystem 205, it being understood that the control system 205 mayincorporate all or part of the diagnostics circuitry 210 or itsfunctionality. The control system 205 may store and use informationprovided by the diagnostics circuitry 210 for a variety of diagnosticspurposes. This diagnostic information may be stored, for example,subsequent to a triggering event or at predetermined intervals, and mayinclude system diagnostics, such as power source status, therapydelivery history, and/or patient diagnostics. The diagnostic informationmay take the form of electrical signals or other sensor data acquiredimmediately prior to therapy delivery.

According to a configuration that provides cardioversion anddefibrillation therapies, the control system 205 processes cardiacsignal data received from the detection circuitry 202 and initiatesappropriate tachyarrhythmia therapies to terminate cardiac arrhythmicepisodes and return the heart to normal sinus rhythm. The control system205 is coupled to shock therapy circuitry 216. The shock therapycircuitry 216 is coupled to the subcutaneous electrode(s) 214 and thecan or indifferent electrode 207 of the ITCS device housing. Uponcommand, the shock therapy circuitry 216 delivers cardioversion anddefibrillation stimulation energy to the heart in accordance with aselected cardioversion or defibrillation therapy. In a lesssophisticated configuration, the shock therapy circuitry 216 iscontrolled to deliver defibrillation therapies, in contrast to aconfiguration that provides for delivery of both cardioversion anddefibrillation therapies. Examples of ICD high energy deliverycircuitry, structures and functionality, aspects of which may beincorporated in an ITCS device of a type that may benefit from aspectsof the present invention are disclosed in commonly owned U.S. Pat. Nos.5,372,606; 5,411,525; 5,468,254; and 5,634,938, which are herebyincorporated herein by reference in their respective entireties.

In accordance with another configuration, an ITCS device may incorporatea cardiac pacing capability in addition to cardioversion and/ordefibrillation capabilities. As is shown in dotted lines in FIG. 1C, theITCS device may include pacing therapy circuitry 230, which is coupledto the control system 205 and the subcutaneous and can/indifferentelectrodes 214, 207. Upon command, the pacing therapy circuitry deliverspacing pulses to the heart in accordance with a selected pacing therapy.Control signals, developed in accordance with a pacing regimen bypacemaker circuitry within the control system 205, are initiated andtransmitted to the pacing therapy circuitry 230 where pacing pulses aregenerated. A pacing regimen may be modified by the control system 205.

A number of cardiac pacing therapies may be useful in a transthoraciccardiac monitoring and/or stimulation device. Such cardiac pacingtherapies may be delivered via the pacing therapy circuitry 230 as shownin FIG. 1C. Alternatively, cardiac pacing therapies may be delivered viathe shock therapy circuitry 216, which effectively obviates the need forseparate pacemaker circuitry.

The ITCS device shown in FIG. 1C is configured to receive signals fromone or more physiologic and/or non-physiologic sensors in accordancewith embodiments of the present invention. Depending on the type ofsensor employed, signals generated by the sensors may be communicated totransducer circuitry coupled directly to the detection circuitry 202 orindirectly via the sensing circuitry 204. It is noted that certainsensors may transmit sense data to the control system 205 withoutprocessing by the detection circuitry 202.

Non-electrophysiological cardiac sensors may be coupled directly to thedetection circuitry 202 or indirectly via the sensing circuitry 204.Non-electrophysiological cardiac sensors sense cardiac activity that isnon-electrophysiological in nature. Examples of non-electrophysiologicalcardiac sensors include blood oxygen sensors, transthoracic impedancesensors, blood volume sensors, acoustic sensors and/or pressuretransducers, and accelerometers. Signals from these sensors aredeveloped based on cardiac activity, but are not derived directly fromelectrophysiological sources (e.g., R-waves or P-waves). Anon-electrophysiological cardiac sensor 261, as is illustrated in FIG.1C, may be connected to one or more of the sensing circuitry 204,detection circuitry 202 (connection not shown for clarity), and thecontrol system 205.

Communications circuitry 218 is coupled to the microprocessor 206 of thecontrol system 205. The communications circuitry 218 allows the ITCSdevice to communicate with one or more receiving devices or systemssituated external to the ITCS device. By way of example, the ITCS devicemay communicate with a patient-worn, portable or bedside communicationsystem via the communications circuitry 218. In one configuration, oneor more physiologic or non-physiologic sensors (subcutaneous, cutaneous,or external of patient) may be equipped with a short-range wirelesscommunication interface, such as an interface conforming to a knowncommunications standard, such as Bluetooth or IEEE 802 standards. Dataacquired by such sensors may be communicated to the ITCS device via thecommunications circuitry 218. It is noted that physiologic ornon-physiologic sensors equipped with wireless transmitters ortransceivers may communicate with a receiving system external of thepatient.

The communications circuitry 218 may allow the ITCS device tocommunicate with an external programmer. In one configuration, thecommunications circuitry 218 and the programmer unit (not shown) use awire loop antenna and a radio frequency telemetric link, as is known inthe art, to receive and transmit signals and data between the programmerunit and communications circuitry 218. In this manner, programmingcommands and data are transferred between the ITCS device and theprogrammer unit during and after implant. Using a programmer, aphysician is able to set or modify various parameters used by the ITCSdevice. For example, a physician may set or modify parameters affectingsensing, detection, pacing, and defibrillation functions of the ITCSdevice, including pacing and cardioversion/defibrillation therapy modes.

Typically, the ITCS device is encased and hermetically sealed in ahousing suitable for implanting in a human body as is known in the art.Power to the ITCS device is supplied by an electrochemical power source220 housed within the ITCS device. In one configuration, the powersource 220 includes a rechargeable battery. According to thisconfiguration, charging circuitry is coupled to the power source 220 tofacilitate repeated non-invasive charging of the power source 220. Thecommunications circuitry 218, or separate receiver circuitry, isconfigured to receive RF energy transmitted by an external RF energytransmitter. The ITCS device may, in addition to a rechargeable powersource, include a non-rechargeable battery. It is understood that arechargeable power source need not be used, in which case a long-lifenon-rechargeable battery is employed.

FIG. 1D illustrates a configuration of detection circuitry 302 of anITCS device, which includes one or both of rate detection circuitry 310and morphological analysis circuitry 312. Detection and verification ofarrhythmias may be accomplished using rate-based discriminationalgorithms as known in the art implemented by the rate detectioncircuitry 310. Arrhythmic episodes may also be detected and verified bymorphology-based analysis of sensed cardiac signals as is known in theart. Tiered or parallel arrhythmia discrimination algorithms may also beimplemented using both rate-based and morphologic-based approaches.Further, a rate and pattern-based arrhythmia detection anddiscrimination approach may be employed to detect and/or verifyarrhythmic episodes, such as by use of the approaches disclosed in U.S.Pat. Nos. 6,487,443; 6,259,947; 6,141,581; 5,855,593; and 5,545,186,which are hereby incorporated herein by reference.

The detection circuitry 302, which is coupled to a microprocessor 306,may be configured to incorporate, or communicate with, specializedcircuitry for processing sensed cardiac signals in manners particularlyuseful in a transthoracic cardiac sensing and/or stimulation device. Asis shown by way of example in FIG. 1D, the detection circuitry 302 mayreceive information from multiple physiologic and non-physiologicsensors. Transthoracic acoustics, for example, may be monitored using anappropriate acoustic sensor. Heart sounds, for example, may be detectedand processed by non-electrophysiologic cardiac sensor processingcircuitry 318 for a variety of purposes. The acoustics data istransmitted to the detection circuitry 302, via a hardwire or wirelesslink, and used to enhance cardiac signal detection and/or arrhythmiadetection. For example, acoustic information may be used in accordancewith the present invention to corroborate ECG rate-based discriminationof arrhythmias.

The detection circuitry 302 may also receive information from one ormore sensors that monitor skeletal muscle activity. In addition tocardiac activity signals, transthoracic electrodes readily detectskeletal muscle signals. Such skeletal muscle signals may be used todetermine the activity level of the patient. In the context of cardiacsignal detection, such skeletal muscle signals are considered artifactsof the cardiac activity signal, which may be viewed as noise. Processingcircuitry 316 receives signals from one or more skeletal muscle sensors,and transmits processed skeletal muscle signal data to the detectioncircuitry 302. This data may be used to discriminate normal cardiacsinus rhythm with skeletal muscle noise from cardiac arrhythmias.

As was previously discussed, the detection circuitry 302 is coupled to,or otherwise incorporates, noise-processing circuitry 314. The noiseprocessing circuitry 314 processes sensed cardiac signals to improve theSNR of sensed cardiac signals by reducing noise content of the sensedcardiac signals.

The components, functionality, and structural configurations depicted inFIGS. 1A–1D are intended to provide an understanding of various featuresand combination of features that may be incorporated in an ITCS device.It is understood that a wide variety of ITCS and other implantablecardiac monitoring and/or stimulation device configurations arecontemplated, ranging from relatively sophisticated to relatively simpledesigns. As such, particular ITCS or cardiac monitoring and/orstimulation device configurations may include particular features asdescribed herein, while other such device configurations may excludeparticular features described herein.

In accordance with embodiments of the invention, an ITCS device may beimplemented to include a subcutaneous electrode system that provides forone or both of cardiac sensing and arrhythmia therapy delivery.According to one approach, an ITCS device may be implemented as achronically implantable system that performs monitoring, diagnosticand/or therapeutic functions. The ITCS device may automatically detectand treat cardiac arrhythmias.

In one configuration, an ITCS device includes a pulse generator and oneor more electrodes that are implanted subcutaneously in the chest regionof the body, such as in the anterior thoracic region of the body. TheITCS device may be used to provide atrial and/or ventricular therapy forbradycardia and tachycardia arrhythmias. Tachyarrhythmia therapy mayinclude cardioversion, defibrillation and anti-tachycardia pacing (ATP),for example, to treat atrial or ventricular tachycardia or fibrillation.Bradycardia therapy may include temporary post-shock pacing forbradycardia or asystole. Methods and systems for implementing post-shockpacing for bradycardia or asystole are described in commonly owned U.S.Patent Application entitled “Subcutaneous Cardiac Stimulator EmployingPost-Shock Transthoracic Asystole Prevention Pacing, Ser. No.10/377,274, filed on Feb. 28, 2003, which is incorporated herein byreference in its entirety.

In one configuration, an ITCS device according to one approach mayutilize conventional pulse generator and subcutaneous electrode implanttechniques. The pulse generator device and electrodes may be chronicallyimplanted subcutaneously. Such an ITCS may be used to automaticallydetect and treat arrhythmias similarly to conventional implantablesystems. In another configuration, the ITCS device may include a unitarystructure (e.g., a single housing/unit). The electronic components andelectrode conductors/connectors are disposed within or on the unitaryITCS device housing/electrode support assembly.

The ITCS device contains the electronics and may be similar to aconventional implantable defibrillator. High voltage shock therapy maybe delivered between two or more electrodes, one of which may be thepulse generator housing (e.g., can), placed subcutaneously in thethoracic region of the body.

Additionally or alternatively, the ITCS device may also provide lowerenergy electrical stimulation for bradycardia therapy. The ITCS devicemay provide brady pacing similarly to a conventional pacemaker. The ITCSdevice may provide temporary post-shock pacing for bradycardia orasystole. Sensing and/or pacing may be accomplished using sense/paceelectrodes positioned on an electrode subsystem also incorporating shockelectrodes, or by separate electrodes implanted subcutaneously.

The ITCS device may detect a variety of physiological signals that maybe used in connection with various diagnostic, therapeutic or monitoringimplementations in accordance with the present invention. For example,the ITCS device may include sensors or circuitry for detecting pulsepressure signals, blood oxygen level, heart sounds, cardiacacceleration, and other non-electrophysiological signals related tocardiac activity. In one embodiment, the ITCS device sensesintrathoracic impedance, from which various respiratory parameters maybe derived, including, for example, respiratory tidal volume and minuteventilation. Sensors and associated circuitry may be incorporated inconnection with an ITCS device for detecting one or more body movementor body position related signals. For example, accelerometers and GPSdevices may be employed to detect patient activity, patient location,body orientation, or torso position.

The ITCS device may be used within the structure of an advanced patientmanagement (APM) system. Advanced patient management systems may allowphysicians to remotely and automatically monitor cardiac and respiratoryfunctions, as well as other patient conditions. In one example,implantable cardiac rhythm management systems, such as cardiacpacemakers, defibrillators, and resynchronization devices, may beequipped with various telecommunications and information technologiesthat enable real-time data collection, diagnosis, and treatment of thepatient. Various embodiments described herein may be used in connectionwith advanced patient management. Methods, structures, and/or techniquesdescribed herein, which may be adapted to provide for remotepatient/device monitoring, diagnosis, therapy, or other APM relatedmethodologies, may incorporate features of one or more of the followingreferences: U.S. Pat. Nos. 6,221,011; 6,270,457; 6,277,072; 6,280,380;6,312,378; 6,336,903; 6,358,203; 6,368,284; 6,398,728; and 6,440,066,which are hereby incorporated herein by reference.

An ITCS device according to one approach provides an easy to implanttherapeutic, diagnostic or monitoring system. The ITCS system may beimplanted without the need for intravenous or intrathoracic access,providing a simpler, less invasive implant procedure and minimizing leadand surgical complications. In addition, this system would haveadvantages for use in patients for whom transvenous lead systems causecomplications. Such complications include, but are not limited to,surgical complications, infection, insufficient vessel patency,complications associated with the presence of artificial valves, andlimitations in pediatric patients due to patient growth, among others.An ITCS system according to this approach is distinct from conventionalapproaches in that it may be configured to include a combination of twoor more electrode subsystems that are implanted subcutaneously in theanterior thorax.

In one configuration, as is illustrated in FIG. 2, electrode subsystemsof an ITCS system are arranged about a patient's heart 510. The ITCSsystem includes a first electrode subsystem, comprising a can electrode502, and a second electrode subsystem 504 that includes at least twoelectrodes or at least one multi-element electrode. The second electrodesubsystem 504 may include a number of electrodes used for sensing and/orelectrical stimulation, and may also include non-electrophysiologicsensors.

In various configurations, the second electrode subsystem 504 mayinclude a combination of electrodes. The combination of electrodes ofthe second electrode subsystem 504 may include coil electrodes, tipelectrodes, ring electrodes, multi-element coils, spiral coils, spiralcoils mounted on non-conductive backing, screen patch electrodes, andother electrode configurations. A suitable non-conductive backingmaterial is silicone rubber, for example.

The can electrode 502 is positioned on the housing 501 that encloses theITCS device electronics. In one embodiment, the can electrode 502includes the entirety of the external surface of housing 501. In otherembodiments, various portions of the housing 501 may be electricallyisolated from the can electrode 502 or from tissue. For example, theactive area of the can electrode 502 may include all or a portion ofeither the anterior or posterior surface of the housing 501 to directcurrent flow in a manner advantageous for cardiac sensing and/orstimulation.

In accordance with one embodiment, the housing 501 may resemble that ofa conventional implantable ICD, is approximately 20–100 cc in volume,with a thickness of 0.4 to 2 cm and with a surface area on each face ofapproximately 30 to 100 cm². As previously discussed, portions of thehousing may be electrically isolated from tissue to optimally directcurrent flow. For example, portions of the housing 501 may be coveredwith a non-conductive, or otherwise electrically resistive, material todirect current flow. Suitable non-conductive material coatings includethose formed from silicone rubber, polyurethane, or parylene, forexample.

In addition, or alternatively, all or portions of the housing 501 may betreated to change the electrical conductivity characteristics thereoffor purposes of optimally directing current flow. Various knowntechniques may be employed to modify the surface conductivitycharacteristics of the housing 501, such as by increasing or decreasingsurface conductivity, to optimize current flow. Such techniques mayinclude those that mechanically or chemically alter the surface of thehousing 501 to achieve desired electrical conductivity characteristics.

As was discussed above, cardiac signals collected from subcutaneouslyimplanted electrodes may be corrupted by noise. In addition, certainnoise sources have frequency characteristics similar to those of thecardiac signal. Such noise may lead to over sensing and spurious shocks.Due to the possibility of relatively high amplitude of the noise signaland overlapping frequency content, filtering alone does not lead tocomplete suppression of the noise. In addition, filter performance isnot generally sufficiently robust against the entire class of noisesencountered. Further, known adaptive filtering approaches require areference signal that is often unknown for situations when a patientexperiences VF or high amplitude noise.

In accordance with one approach of the present invention, an ITCS devicemay be implemented to discriminate cardiac signals within a group ofseparated signals, such as those obtained from a blind source separation(BSS) technique. Devices and methods of blind source separation arefurther described in commonly owned U.S. patent application Ser. No.10/741,814, filed Dec. 19, 2003, hereby incorporated herein byreference. Devices and methods associated with another useful signalseparation approach that uses noise canceling electrodes are furtherdescribed in commonly owned U.S. patent application Ser. No. 10/738,608,filed Dec. 17, 2003, hereby incorporated herein by reference.

Information from a non-electrophysiologic sensor 503, such as thosedescribed previously, may be used to improve the accuracy of arrhythmiadiscrimination, such as ECG or other rate-based arrhythmiadiscrimination. A signal independent of cardiac electrical activity,such as an acoustic signal of cardiac heart-sounds, an accelerometer, ablood sensor, or other non-electrophysiologic source sensor, may be usedto improve the detection and discrimination of arrhythmia from normalsinus rhythm (NSR) in the presence of noise. The non-electrophysiologicsensor 503 may be provided in or on the housing 501, as illustrated inFIG. 2, or may be provided as part of the second electrode subsystem504, as described earlier. It is also contemplated that thenon-electrophysiologic sensor 503 may be directly coupled to the housing501 using an additional lead, or be wirelessly coupled as described withreference to FIGS. 1C and 1D.

In an embodiment of the present invention, heart sounds are used to aidin signal discrimination when detecting various heart rhythms in thepresence of electrical noise and/or electrocardiographic artifacts.Because the additional discriminating non-electrophysiologic signal istime correlated with respect to the cardiac electrophysiologicalsignals, the non-electrophysiologic signal may provide information abouta patient's rhythm state even in the presence of electrical noise and/orelectrocardiographic artifacts. For example, the non-electrophysiologicsignal may be used to verify that the ECG signal contains a cardiacsignal having a QRS complex, and only ECG signals with QRS complexes areverified ECG signals. Subsequent analysis may require that only verifiedECG signals are used for calculations of, for example, heart rate. Thisprovides for more robust algorithms that are less susceptible tocontamination from electrical interference and noise.

In one embodiment, a subcutaneous sensor, such as an accelerometer oracoustic transducer, may be used to detect heart sounds. The heartsounds may be used together with rate, curvature, and other ECGinformation to discriminate normal sinus with electrical noise frompotentially lethal arrhythmias such as ventricular tachycardia andventricular fibrillation. An ITCS device may utilize one or more of thepresence, characteristics, and frequency of occurrence of the heartsound combined with ECG information when performing signal or rhythmdiscrimination.

A heart rate determined from the ECG signal may, for example, beanalyzed along with heart sound information for diagnostic purposes.High ECG heart rate detection along with normal rate heart sounds wouldindicate the presence of noise in the ECG signal. High ECG heart ratedetection along with modified heart sounds would indicate a potentiallylethal arrhythmia. It is noted that ECG morphology or other techniquescould replace rate in the example above. It should also be noted thatother sensor derived signals could replace heart sounds. For example,impedance, pulse pressure, blood volume/flow, or cardiac accelerationscould be used.

Various types of acoustic sensors may be used to detect heart sounds.Examples of such acoustic sensors include diaphragm based acousticsensors, MEMS-based acoustic sensors such as a MEMS-based acoustictransducer, fiber optic acoustic sensors, piezoelectric sensors, andaccelerometer based acoustic sensors and arrays. These sensors may beused to detect the audio frequency pressure waves associated with theheart sounds, and may also be used to detect othernon-electrophysiologic cardiac related signals.

The presence of cardiac pulse, or heartbeat, in a patient is generallydetected by palpating the patient's neck and sensing changes in thevolume of the patient's carotid artery due to blood pumped from thepatient's heart. A graph of a carotid pulse signal 810, representativeof the physical expansion and contraction of a patient's carotid arteryduring two consecutive pulses, or heartbeats, is shown at the top ofFIG. 3. When the heart's ventricles contract during a heartbeat, apressure wave is sent throughout the patient's peripheral circulationsystem. The carotid pulse signal 810 shown in FIG. 3 rises with theventricular ejection of blood at systole and peaks when the pressurewave from the heart reaches a maximum. The carotid pulse signal 810falls off again as the pressure subsides toward the end of each pulse.

The opening and closing of the patient's heart valves during a heartbeatcauses high-frequency vibrations in the adjacent heart wall and bloodvessels. These vibrations can be heard in the patient's body as heartsounds, and may be detected by sensors, as described earlier. Aconventional phonocardiogram (PCG) transducer placed on a patientconverts the acoustical energy of the heart sounds to electrical energy,resulting in a PCG waveform 820 that may be recorded and displayed, asshown by the graph in the upper middle portion of FIG. 3.

As indicated by the PCG waveform 820 shown in FIG. 3, a typicalheartbeat produces two main heart sounds. A first heart sound 830,denoted S1, is generated by vibration generally associated with theclosure of the tricuspid and mitral valves at the beginning of systole.Typically, the heart sound 830 is about 14 milliseconds long andcontains frequencies up to approximately 500 Hz. A second heart sound840, denoted S2, is generally associated with vibrations resulting fromthe closure of the aortic and pulmonary valves at the end of systole.While the duration of the second heart sound 840 is typically shorterthan the first heart sound 830, the spectral bandwidth of the secondheart sound 840 is typically larger than that of the first heart sound830.

An electrocardiogram (ECG) waveform 850 describes the electricalactivity of a patient's heart. The graph in the lower middle portion ofFIG. 3 illustrates an example of the ECG waveform 850 for two heartbeatsand corresponds in time with the carotid pulse signal 810 and PCGwaveform 820 also shown in FIG. 3. Referring to the first shownheartbeat, the portion of the ECG waveform 850 representingdepolarization of the atrial muscle fibers is referred to as the “P”wave. Depolarization of the ventricular muscle fibers is collectivelyrepresented by the “Q.” “R,” and “S” waves of the ECG waveform, referredto as the QRS complex. Finally, the portion of the waveform representingrepolarization of the ventricular muscle fibers is known as the “T”wave. Between heartbeats, the ECG waveform 850 returns to anisopotential level.

Fluctuations in a patient's transthoracic impedance signal 860 alsocorrelate with blood flow that occurs with each cardiac pulse wave. Thebottom graph of FIG. 3 illustrates an example of a filteredtransthoracic impedance signal 860 for a patient in which fluctuationsin impedance correspond in time with the carotid pulse signal 810, thePCG waveform 820, and ECG waveform 850, also shown in FIG. 3.

Referring now to FIG. 4, in another embodiment of the present inventioninvolving heart sounds, such sounds may be used for discrimination ofarrhythmia from normal sinus rhythm. FIG. 4 is a graph depicting twoconsecutive PQRS complexes in the ECG signal 850 and their associatednon-electrophysiological components developed from an accelerometersignal 835. Also illustrated is a detection window 870 that is used toevaluate correlation of the signals in accordance with an embodiment ofthe present invention. As is illustrated in FIG. 4, an S1 heart sound832, and an S1 heart sound 834 are, in general, closely time correlatedwith a QRS complex 852 and a QRS complex 854 respectively. The S1 heartsound 832, an S2 heart sound 833, and the S1 heart sound 834 areillustrated as detected from an internally implanted accelerometer. TheS1 heart sound may provide a close time correlation with cardiac signalsbut not with noise and artifact signals. As such, heart sounds may beused to discriminate an arrhythmia from NSR.

In an embodiment of a method in accordance with the present invention, amethod of arrhythmia detection uses the ECG signal to define a detectionwindow. A non-electrophysiological source signal is then evaluatedwithin the detection window for cardiac information. If thenon-electrophysiological source signal includes a cardiac event withinthe window, then the ECG signal is corroborated as corresponding to acardiac event. This may be used, for example, in a rate-based arrhythmiadetection algorithm to provide a more robust rate than the ratecalculated if only ECG information is used. The algorithm may, forexample, only count ECG identified heart beats if the heart beats arecorroborated by an associated non-electrophysiologically sensed heartbeat.

An ITCS device may be implemented to include signal processing circuitryand/or signal processing software as illustrated in FIGS. 1C and 1D.With continued reference to FIG. 4, signal processing may be used tocorrelate heart sounds, such as the S1 heart sound, with R-wave peaks orother QRS complex features to provide discrimination of arrhythmias fromNSR in the presence of noise.

In the approach illustrated in FIG. 4, an examination or detectionwindow 870 is defined to start at a start time 875, based on the Q pointof the QRS complex 852. The ITCS algorithm then searches theaccelerometer signal 835 within the detection window 870 for the S1heart sound 832. The algorithm may also look for time correlationbetween peak amplitudes of the S1 heart sound 832 and the peak R of theQRS complex 852. For example, the ECG signal 850 has an R-wave peak 856falling within the examination window 870, and an R-wave peak 858falling within an examination window 872. The R-wave peak 856 fallingwithin the examination window 870 produces a large correlation value,indicating that the ECG signal 850 is time correlated to the S1 heartsound signal 832 within the examination window 870. Similarly, theR-wave peak 858 falling within the examination window 872 produces alarge correlation value, indicating that the ECG signal 850 is timecorrelated to the S1 heart sound signal 834 within the examinationwindow 872. Heart rate, for example, may be determined betweensuccessive heart beats with large correlation values of the QRScomplexes 852, 854 and their associated S1 heart sounds 832, 834.

Referring now to FIG. 5, methods of signal discrimination in accordancewith the present invention are illustrated in a flowchart 900.Electrocardiogram signals 902 are received at a subcutaneousnon-intrathoracic location. The electrocardiogram signals 902 mayinclude a cardiac signal and one or both of noise andelectrocardiographic artifacts. Non-electrophysiologic signals 904associated with a non-electrophysiological cardiac source are alsoreceived to provide cardiac function information that isnon-electrophysiologic in nature, such as heart sound information, bloodflow information, blood oxygen information, and information from othernon-electrophysiologic sensors described earlier. Both theelectrocardiogram signals 902 and non-electrophysiologic signals 904 areused to discriminate between normal sinus rhythm and an arrhythmia viaseveral optional paths, as is illustrated in the flowchart 900 of FIG.5.

An arrhythmia may be detected using the electrocardiogram signals 902,and the presence of the arrhythmia may be verified using a comparison903 of the electrocardiogram signals 902 to the non-electrophysiologicsignals 904. Temporal relationships between the electrocardiogramsignals 902 and non-electrophysiologic signals 904 may be determinedsuch as by using a comparison 905 of morphologies 907,909 of theelectrocardiogram signals 902 and the non-electrophysiologic signals 904respectively.

A detection window 906 may be initiated in response to receiving theelectrocardiogram signal 902, and used to determine whether thenon-electrophysiologic signal 904 is received at a time falling withinthe detection window 906, such as by using a correlation 911. The starttime of the detection window 906 used for confirmation may be associatedwith an inflection point of the electrocardiogram signal, such as amaxima or a minima or any other suitable morphological attribute.

Heart rates may be computed based on both a succession ofelectrocardiogram signals 902 and a succession of non-electrophysiologicsignals 904. An ECG rate 908 and a signal rate 916 may be used todiscriminate between normal sinus rhythm and the arrhythmia. The ECGrate 908 may be compared with an arrhythmia threshold 910, and used todetermine presence/absence of an arrhythmia, such as in response to afirst rate exceeding a first arrhythmia threshold but having a signalrate 916, at a second rate, failing to exceed a second arrhythmiathreshold 918.

Using any path in the flow-chart 900, defibrillation therapy deliverymay be inhibited 920 or treated 921 in response to detecting anarrhythmia using the electrocardiogram signals 902 and confirming orrejecting the arrhythmia using, for example, the comparison 903, acomparison 941, and/or the correlation 911.

FIG. 6 is a flow chart illustrating a method of multi-parameterarrhythmia discrimination in accordance with another embodiment of thepresent invention. An arrhythmia discrimination method 950 isillustrated that involves to sensing 951 an electrocardiogram signal ata subcutaneous non-intrathoracic location. A signal associated with anon-electrophysiological cardiac source is received 952 and subject toverification 953 to determine whether or not the sensedelectrocardiogram signal includes a cardiac signal. A cardiac arrhythmiais detected 954 using one of the sensed electrocardiogram signal and theverified cardiac signal. Treatment of the cardiac arrhythmia is withheld955 if the sensed signal is not the cardiac signal. A verificationmethodology according to this and other embodiments advantageouslyreduces or eliminates delivery of unnecessary cardiac shocks, byensuring that the sensed signal from which arrhythmia detection,confirmation, and therapy decisions are made is indeed a cardiac signal.

Approaches to cardiac signal discrimination described herein involve theuse of a non-electrophysiologic signal to discriminate and/or verify anarrhythmia and its associated ECG signal. Because signals developed fromnon-electrophysiological cardiac sources, such as heart sounds, are notelectrophysiological in nature, they are not susceptible to the samenoise sources as electrocardiogram signals.

An ITCS device may operate in a batch mode or adaptively, allowing foron-line or off-line implementation. To save power, the system mayinclude the option for a hierarchical decision-making routine that usesalgorithms known in the art for identifying presence of arrhythmias ornoise in the collected signal and judiciously turning on and off thecardiac signal discrimination methods in accordance with the presentinvention.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. An arrhythmia discrimination method, comprising: sensingelectrocardiogram signals at a subcutaneous non-intrathoracic location;receiving signals associated with a non-electrophysiological cardiacsource, the received signals comprising at least one of heart soundsignals, pulse oximetry signals, impedance signals indicative of cardiacactivity, acoustic emission information, blood-flow information, andheart rate information; verifying that the electrocardiogram signalscomprise a cardiac signal using the non-electrophysiological signal;discriminating, using the electrocardiogram signals andnon-electrophysiologic signals, between a normal sinus rhythm and acardiac arrhythmia; and withholding delivery of subcutaneousnon-intrathoracic cardiac stimulation therapy if the sensedelectrocardiogram signals do not comprise the cardiac signal.
 2. Themethod of claim 1, wherein discriminating between normal sinus rhythmand the arrhythmia comprises: detecting the arrhythmia using theelectrocardiogram signals; and verifying presence of the arrhythmiausing the non-electrophysiologic signals.
 3. The method of claim 1,wherein discriminating between normal sinus rhythm and the arrhythmiacomprises: detecting the arrhythmia using the electrocardiogram signals;determining temporal relationships between the electrocardiogram signalsand non-electrophysiologic signals received while detecting thearrhythmia; and verifying presence of the arrhythmia based on thetemporal relationships between the electrocardiogram signals andnon-electrophysiologic signals.
 4. The method of claim 1, whereindiscriminating between normal sinus rhythm and the arrhythmia comprises:initiating a detection window in response to receiving eachelectrocardiogram signal of a succession of the electrocardiogramsignals; and determining whether each non-electrophysiologic signal of asuccession of the non-electrophysiologic signals is received at a timefalling within the detection window.
 5. The method of claim 1, whereindiscriminating between normal sinus rhythm and the arrhythmia comprises:computing a first rate based on successive electrocardiogram signals;computing a second rate based on successive non-electrophysiologicsignals; and discriminating between normal sinus rhythm and thearrhythmia using the first and second rates.
 6. The method of claim 1,wherein discriminating between normal sinus rhythm and the arrhythmiacomprises: computing a first rate based on successive electrocardiogramsignals; computing a second rate based on successivenon-electrophysiologic signals; comparing the first rate with a firstarrhythmia threshold; comparing the second rate with a second arrhythmiathreshold; and determining presence of the arrhythmia in response toboth the first and second rates exceeding the first and secondarrhythmia thresholds, respectively.
 7. The method of claim 1, whereindiscriminating between normal sinus rhythm and the arrhythmia comprises:computing a first rate based on successive electrocardiogram signals;computing a second rate based on successive non-electrophysiologicsignals; comparing the first rate with a first arrhythmia threshold;comparing the second rate with a second arrhythmia threshold; anddetermining absence of the arrhythmia in response to the first rateexceeding the first arrhythmia threshold and the second rate failing toexceed the second arrhythmia threshold.
 8. The method of claim 1,wherein discriminating between normal sinus rhythm and the arrhythmiacomprises: determining presence of the arrhythmia using a morphology ofthe electrocardiogram signals; and verifying presence of the arrhythmiausing the non-electrophysiologic signals.
 9. The method of claim 1,wherein the non-electrophysiologic signals comprise subsonic acousticsignals indicative of cardiac activity.
 10. The method of claim 1,wherein the non-electrophysiologic signals comprise pulse pressuresignals.
 11. The method of claim 1, further comprising declaring anarrhythmic episode in response to detecting the arrhythmia using theelectrocardiogram signals and detecting the arrhythmia using thenon-electrophysiologic signals.
 12. The method of claim 1, furthercomprising enabling defibrillation therapy delivery in response todetecting the arrhythmia using the electrocardiogram signals anddetecting the arrhythmia using the non-electrophysiologic signals. 13.The method of claim 1, further comprising inhibiting defibrillationtherapy delivery in response to detecting the arrhythmia using theelectrocardiogram signals but not detecting the arrhythmia using thenon-electrophysiologic signals.
 14. An arrhythmia discrimination method,comprising: sensing an electrocardiogram signal at a subcutaneousnon-intrathoracic location; receiving a signal associated with anon-electrophysiological cardiac source; verifying that the sensedelectrocardiogram signal comprises a cardiac signal using thenon-electrophysiological cardiac signal; detecting a cardiac arrhythmiausing one of the sensed electrocardiogram signal and the verifiedcardiac signal; confirming the detection of the cardiac arrhythmia byperforming a correlation between the electrocardiogram signal and thenon-electrophysiological cardiac signal; and withholding treatment ofthe cardiac arrhythmia if the sensed electrocardiogram signal does notcomprise the cardiac signal.
 15. The method of claim 14, furthercomprising: defining a detection window with a start time associatedwith an inflection point of the electrocardiogram signal; and evaluatingthe received non-electrophysiological cardiac signal within thedetection window.
 16. The method of claim 15, wherein the start time ofthe detection window is associated with a maxima or a minima of theelectrocardiogram signal.
 17. The method of claim 14, furthercomprising: computing a first heart-rate based on intervals betweensuccessive electrocardiogram signals; and computing a second heart-ratebased on intervals between successive non-electrophysiological cardiacsignals; wherein confirming the detection of the cardiac arrhythmiacomprises comparing the first heart-rate to the second heart-rate. 18.The method of claim 14, wherein the non-electrophysiological cardiacsignal comprises acoustic emission information.
 19. The method of claim18, wherein the acoustic emission information comprises a temporallocation of a peak heart-sound.
 20. The method of claim 14, wherein thenon-electrophysiological cardiac signal comprises cardiac accelerationinformation.
 21. The method of claim 14, wherein thenon-electrophysiological cardiac signal comprises pulse pressureinformation.
 22. The method of claim 14, wherein thenon-electrophysiological cardiac signal comprises blood-flowinformation.
 23. The method of claim 14, wherein thenon-electrophysiological cardiac signal comprises heart rateinformation.
 24. The method of claim 14, wherein thenon-electrophysiological cardiac signal comprises pulse oximetryinformation.
 25. The method of claim 14, wherein detecting the cardiacarrhythmia comprises performing a rate based analysis of theelectrocardiogram signal.
 26. The method of claim 14, wherein detectingthe cardiac arrhythmia comprises performing a morphology based analysisof the electrocardiogram signal.
 27. The method of claim 14, furthercomprising delivering a cardiac therapy to treat the cardiac arrhythmia.28. An implantable cardiac device, comprising: an implantable housing;an electrode arrangement configured for subcutaneous non-intrathoracicplacement; detection circuitry provided in the housing and coupled tothe electrode arrangement, the detection circuitry configured to detectelectrocardiogram signals; a sensor configured to sense signalsassociated with a non-electrophysiological cardiac source, the sensorcomprising at least one of a microphone, acoustic transducer, blood-flowtransducer, pulse oximeter, and photoplethysmography circuitry; energydelivery circuitry coupled to the electrode arrangement; and a processorprovided in the housing and coupled to the detection circuitry, sensor,and energy delivery circuitry, the processor using thenon-electrophysiological signals to verify that the detectedelectrocardiogram signals comprise a cardiac signal, the processorwithholding energy delivery if the detected electrocardiogram signals donot comprise the cardiac signal.
 29. The device of claim 28, wherein theenergy delivery circuitry comprises defibrillation therapy circuitry.30. The device of claim 28, wherein the energy delivery circuitrycomprises pacing therapy circuitry.
 31. The device of claim 28, whereinthe sensor is provided in or on the housing.
 32. The device of claim 28,wherein the sensor is provided in or on a lead coupled to the housing.33. The device of claim 28, wherein the sensor comprises anaccelerometer.